\x is just a hexadecimal escape sequence. It has nothing to do with encodings on its own. US-ASCII goes from "\x00" to "\x7F" (e.g. "\x41" is the same as "A", "\x30" is "0"). The rest ("\x80" to "\xFF") however are not US-ASCII characters since it's a 7-bit character set.

If you want to check if a string contains only US-ASCII characters, call String#ascii_only?: